1. Field of the Invention
The present invention relates to novel optically-active aliphatic .beta.-halogen substituted carboxylic acid 4'-(4-alkoxybenzyloxy)biphenyl thioester compounds defined by the general formula: ##STR2## wherein R represents a straight-chained alkyl radical having C.sub.2 to C.sub.10 carbon atoms and X represents a halogen atom, and a process for the preparation thereof.
In particular, the present invention relates to novel compounds usable as a base material or a dopant for liquid crystal blending to improve the properties of LCD, and a process for the preparation thereof.
2. Description of the Prior Art
Generally, a liquid crystal phase having an optically-active group has been well-known but the fact that a portion of a smectic phase shows ferroelectricity was first discovered by R. M. Meyer.
Since this ferroelectric liquid crystal (FLC) has desired properties, such as fast response time and bistability, many specialists have taken an interest in FLC in order to apply it to color TVs, computer terminals, electrically-optical modulators, etc., utilizing such properties.
However, studies on FLC have been stagnated due to the difficulties of its applicability. After a surface stabilized ferroelectric liquid crystal (SSFLC) was made public it has become a more important material. Accordingly, various theoretical, experimental and chemical studies on FLC have been carried out in its field of application.
Since an FLC material having fast response time and bistability and being a smectic phase (Sc) in a broad temperature range is necessary in order to apply an FLC to liquid crystal display elements, several classes of an FLC substance have been synthesized to meet such conditions. In spite of all efforts, with such conventional FLC substance there is much left to be desired.
An FLC substance of cinnamate and phenyl benzoate series having the chain of ##STR3## was disclosed. Recently others have studied biphenyl and benzoate series having a chiral center which can be obtained from optically-active .alpha.-amino acid. From their results, they explained that these compounds show high spontaneous polarization by introducing two adjacent asymmetric carbon atoms into the side of dipole moment and the magnitude of the spontaneous polarization depends on the size of an atom bonding to a carbon atom and the polarity.
In 1988, it was indicated by studies on p-n-alkoxy-.alpha.-methyl cinnamic acid ester derivatives, that a halogen substituent of an asymmetric carbon atom induces high spontaneous polarization by introducing a chlorine substituent into .alpha.-asymmetric carbon atom of optically-active group.
It has been found that biphenyl series are chemically stable and an ester bridge between phenyl groups has a tendency to lower the transition temperature. It has also been found that two adjacent asymmetric carbon atoms at the side of dipole moment and a halogen substituent of an asymmetric carbon atom induce high spontaneous polarization.
Thus, in a ferroelectric liquid crystal substance:
1. a rigid core
1) is a primary basic anisotropic unit, and PA2 2) is a complex cyclic group as well; PA2 1) is an essential member to form liquid crystals, PA2 2) helps to form a declined smectic phase, and PA2 3) has an effect on the transition temperature by the size of its chain; and PA2 1) should be as large as possible because of its effect on formation of an FLC mesophase, and PA2 2) should be close to a chiral center to induce high spontaneous polarization.
2. a flexible end chain
3. the transverse dipole
On the basis of the above, a molecular design is constructed as follows: ##STR4##
By these basic conditions, the properties of substantially practical FLC substance are as follows:
1. it should exhibit a smectic phase in the practical temperature range; PA1 2. it should be chemically stable; PA1 3. its spontaneous polarization should be high; and PA1 4. its bistability should be good and response speed high.